a) Field of the Invention
The present invention relates to a method of detecting a relative position of a mask and a wafer, and to a position detecting mark (an alignment mark). More particularly, the invention relates to a position detecting method (an alignment method) suitable for improving throughput of proximity exposure, and to a position detecting mark.
b) Description of the Related Art
A vertical detection method and an oblique detection method are known as a method of detecting the positions of a wafer and a mask by using an aligner having a lens system combined with an image processing system. The vertical detection method observes an alignment mark along a direction perpendicular to the mask plane, and the oblique detection method observes it obliquely.
A chromatic bifocal method is known as a focussing method used by the vertical detection method. The chromatic bifocal method observes alignment marks formed on a mask and a wafer by using light of different wavelengths and chromatic aberrations of the lens system, and focuses the images of the masks on the same flat plane. An absolute precision of position detection by the chromatic bifocal method can be made high because the optical resolution of the lens system can be set high in principle.
However, since an alignment mark is observed vertically, a part of the optical system enters the exposure area. Since the optical system shields exposure light, it is necessary to retract the optical system from the exposure area when exposure light is applied. A time required for retracting the optical system lowers throughput. The alignment mark cannot be observed during the exposure, which is one of the reasons of lowering an alignment precision during the exposure.
With the oblique detection method, the optical axis of the optical system is disposed obliquely to the mask plane, and the system can be disposed without shielding the exposure system. It is therefore unnecessary to extract the optical system during exposure, permitting observation of an alignment mark even during the exposure. Therefore, throughput does not lower and position misalignment during the exposure can be prevented.
A conventional oblique detection method uses oblique focussing in which regular reflection light reflected from the mark is obliquely focussed to detect the image of the mark. An absolute precision of position detection is therefore lowered by image distortion. Furthermore, since regular reflection light is incident to an observation lens, the optical axis of illumination light cannot coincide with the optical axis of observation light. Since the optical axes of illumination and observation light are required to be separated, if there is even a slight shift between both the axes, the detection precision is lowered and the installation of the optical system becomes complicated.